Small waterplane twin hull vessels generally consist of two submerged hulls, originally formed of uniform cross-section, connected to a work platform or upper hull by elongated struts which have a cross-section substantially smaller than the cross-section of the submerged hulls. It is for that reason that such vessels are characterized as "small waterplane twin hull" vessels.
Originally SWATH vessels utilized single struts between the two submerged hulls and the upper platform, as shown, for example, in U.S. Pat. No. 3.447,502, issued to Leopold and U.S. Pat. No. 4,552,083 issued to Schmidt. Some time ago, however, the Naval Ocean System Center at San Diego and Honolulu developed a new SWATH design characterized by having at least two struts associated with each submerged hull. These vessels were characterized by submerged twin hulls of uniform cross section with at least two narrow struts making the connection at the forward and aft ends of the submerged hulls and the upper platform. These struts typically extended vertically, as shown, for example, in U.S. Pat. Nos. 3,623,444 and 3,897,944, issued to Lang.
It has been found that SWATH vessels having multiple struts having better operational characteristics than conventional ships and can operate at much higher sea states.
Subsequent to these developments other improvements were made in SWATH vessels. For example, U.S. Pat. No. 4,174,671 to Seidl taught that the submerged hulls can have non-uniform cross-sections in order to obtain improved operational characteristics. U.S. Pat. No. 4,944,238 to Lang shows that the struts may have varying shapes along their length to improve handling characteristics in high seas. That patent, in FIG. 5, shows the concept of a negatively canted strut on a multi-strut vessel. Negative canting in this way produces a very wide vessel or a very narrow deck for the vessel.
In U.S. Pat. Nos. 4,557,211 and 4,798,153, Schmidt suggests canting single struts on a SWATH vessel outwardly or inclining the submerged hulls upwardly in order to counterbalance the tendency of SWATH vessels to run in a bow down condition. That kind of trim condition decreases the ship's stability and increases the possibility of propeller broaching and ventilating. Schmidt's designs were an attempt to correct such trim conditions and to avoid the necessity for additional power required in vessels such as shown in the above-mentioned Lang patents that provide trim control surfaces which increase drag on the vessel.